Melt grown alumina crystals and process therefor

ABSTRACT

MASSIVE ALPHA ALUMINA UNICRYSTALLINE MATERIAL SUBSTANTIALLY FREE OF INTERNAL STRAIN AND CRYSTAL MISORIENTATION IS PROVIDED. A PROCESS FOR PRODUCING SUCH MATERIAL BY PULLING FROM AN ALUMINA MELT IS ALSO PROVIDED.   D R A W I N G

Feb. 6, 1973 M. N. PLOOSTER 3,715,194

MELT GROWN ALUMINA CRYSTALS AND PROCESS THEREFOR 2 Sheets-Sheet 1Original Filed Aug. 20, 1965 INVENTOR MYRON N. PLOOSTER ATTORNEY Fab. 6,1973 M. N. PLOOSTER Mllbl' GROWN ALUMINA CRYSTALS AND PROCESS THEREFOROriginal Filed Aug. 20, 1965 2 Sheets-Sheet 2 SATURABLE CORE REACTORINDUCTION c HEATING 10 MAGNETIC POWER AMPLIFIER SUPPLY /4 q q'-EPLfi&TEENT -RADIATION u SHUNT PYROMETER TEMPERATURE CONTROL POWER -o-CONTROL\ t I V i SET POINT DEVIATION UNIT AMPUF'IER CONTROLLER INVENTORMYRON N.PLOOSTER kur A ATTORNEY United States Patent Oflice 3,715,194Patented Feb. 6, 1973 Int. c1. BOlg 17/18; Clf 7/02 US. Cl. 23301 SP 7Claims ABSTRACT OF THE DISCLOSURE Massive alpha alumina unicrystallinematerial substantially free of internal strain and crystalmisorientation is provided. A process for producing such material bypulling from an alumina melt is also provided.

This application is a division of copending application Ser. No.770,894, filed Oct. 22, 1968, and now abandoned which is in turn acontinuation of application Ser. No. 481,l94, filed Aug. 20, 1965 andnow abandoned, which is in turn a continuation-in-part of applicationSer. No. 300,179, filed Aug. 6, 1963 and now abandoned.

This invention relates to a novel alpha alumina unicrystalline materialwhich is substantially free of internal strain and crystalmisorientations in the as-grown state and to a process for obtainingsuch material by pulling from an alumina melt. A particular embodimentof the instant invention relates to a novel unicrystalline ruby materialand to a process for obtaining such material by pulling a crystal from amelt of alumina and chromia.

Unicrystalline water-white alpha alumina material and unicrystallinecolored alpha alumina material containing such colorants as chromia(red), titania and iron oxide (blue) or oxides such as those ofmanganese, cobalt, vanadium and nickel (other colors), for example, areknown in the prior art. Such materials were usually grown by theVerneuil technique wherein flame-fused powdered material was droppedonto the molten cap of a seed rod. The feed material was then solidifiedalong the bottom of the molten cap so as to increase the length of theseed rod. The seed rod was slowly lowered so as to maintain the moltensurface of the cap at a substantially constant distance from the heatsource. This prior art technique was especially useful for preparationof unicrystalline product material having an elongated form, such as arod. Other prior art techniques for growing crystals of alpha aluminaand ruby include the fused salt flux, hydrothermal and vapor processes.

The fiame-Verneuil technique has been known for about 50 years and hasbeen the technique previously employed to obtain massive non-granularunicrystalline bodies of refractory materials such as alpha alumina(corundum) and ruby; other prior art techniques were not capable ofproducing large unicrystalline bodies of such material.

Large unicrystalline ruby is used in optical maser or laserapplications. Briefly, lasers operate on the principle of lightamplification and can create extremely intense concentrations of light.Generally, the ruby laser material is in the form of an elongated rod.The light beam produced by the laser is transmitted through the ruby rodand passes out through an end.

The structure of the ruby laser material must be very nearly perfectsince any optical inhomogeneities will cause distortion of the laserbeam and thereby destroy the coherence of the beam. Imperfections in theruby crystal which adversely affect lasing performance includemisorientations, chromium concentration gradients, dislocations,inclusions and bubbles.

It is the principal object of the present invention to provide massivenon-granular unicrystalline ruby material which is substantially free ofdislocations, crystal misorientations, inclusions, bubbles, internalstrain, and chromium gradiations.

It is a further object of the present invention to provide massivenon-granular unicrystalline alpha alumina material which issubstantially free of dislocations, crystal misorientations, inclusions,bubbles, and internal strain.

It is still another object of the present invention to provide a processfor the preparation of massive nongranular unicrystalline ruby materialwhich is substantially free of dislocations, crystal misorientations,inclusions, bubbles, internal strain and chromium gradiations.

It is still a further object of the present invention to provide aprocess for the preparation of massive nongranular unicrystalline alphaalumina material which is substantially free of dislocations,inclusions, bubbles and internal strain.

The term massive non-granular," as used herein, is intended to designatesingle crystals as distinguished from a sintered or agglomerated mass oftiny granules. Moreover, this term is further intended to designatecrystals which are larger than such tiny particles or granules and whichare large enough to be used in optical equipment and, in particular, inlaser equipment.

The above referred to objects, other objects, features and advantages ofthe present invention will become more apparent from the followingdescription, appended claims and drawings in which:

FIG. 1 is an elevational view, partly in section, of the apparatus forcarrying out the process and producing the novel unicrystallinematerials of ruby and alpha alumina of this invention.

FIG. 2 is a block diagram of the control circuit used in conjunctionwith the apparatus of FIG. 1 for carrying out the objects of thisinvention.

Briefly, and in accordance with one aspect of the present invention,massive unicrystalline ruby material is produced having a minimum linealcross sectional dimension of A of an inch and a minimum lineallongitudinal dimension of 2 inches, and a dislocation density notexceeding 10,000 C./cm. The preferred massive unicrystalline rubymaterial of this invention is further characterized by misorientationsmeasuring between 5 and ll) seconds of arc, no inclusions, substantiallyno bubbles or internal strain, and a very low incidence of chromiumvariation in any direction.

Briefly, and in accordance with another aspect of this invention, rubyis produced by the immersion of a seed material such as sapphire or rubyin a melt of ruby and gradually withdrawn from the melt at a rate notexceeding 0.50 inch per hour. The temperature of the ruby melt is heldsubstantially constant except when regulation of the diameter of thegrowing crystal is necessary, and the thermal gradients within the meltare controlled such that they are substantially constant and ofrelatively low value. The atmosphere over the melt is inert to the meltalthough it may be selectively made either slightly reducing or slightlyoxidizing. The melt is maintained contaminant free. Thus, the means bywhich the melt is maintained molten and its containing vessel are suchthat they will not be a source of contamination.

Briefly, and in accordance with still another aspect of the presentinvention, alumina starting material containing any desired additive,such as chromia, is placed in a suitable refractory container orcrucible and heated until it is molten. The container or crucible isconstructed of refractory material having a melting point higher thanthe melting point (2040 C.) of the alumina. Additionally, the crucibleshould be able to withstand high thermal shock created by the moltenalumina and be substantially chemically inert to the molten alumina.Tungsten and iridium have been found useful as crucible materials forthis service with iridium being preferred. When tungsten is employed asthe crucible material, it is preferred that an inert atmosphere bemaintained around the crucible to prevent loss of tungsten due tooxidation. In any event. an inert atmosphere is preferred regardless ofthe crucible material inasmuch as the highest quality crystals areproduced with such an atmosphere. Such atmosphere can contain such inertgases as. for example, argon, helium, neon or krypton or nitrogen, whichappears to be relatively inert in this application. Heating of thealumina starting material to form the melt is preferably accomplished byinductive electrical heating. Other methods of heating can be employed,however, if they are readily controllable and do not contaminate thegrowing environment of the crystal. In the inductive heating techniquethe crucible is employed as a susceptor in an R-F alternating electricfield. Currents are induced in the susceptor crucible and thus heat thecrucible to a high temperature whereby the contained alumina is heatedby conduction. Inductive heating can be employed at atmospheric pressureor at pressures above or below atmospheric. Alternatively, the cruciblecan be heated by direct application of electrical potential and thuscause resistance currents to pass through the crucible. A flame, such asan oxygen-hydrogen combustion flame. can also be directed at thecrucible. Also an electrical arc can be struck to the crucible and thecrucible heated by resistance current. Still another technique is theuse of an arc-heated gas stream, such as that obtained by anon-transferred arc torch described in US. 2,858,411 to heat thecrucible. In all these heating techniques the alumina starting materialis heated by conduction from the hot crucible. It is of extremeimportance in all these heating techniques to guard againstcontaminating the crystal growing environment and means must be providedas well to allow for the introduction and maintenance of the desiredatmosphere above the melt. Once the alumina charge becomes molten,convection currents within the melt tend to stir the melt and even outthe melt composition. A unicrystalline seed having desired compositionand desired crystal orientation is then placed in contact with thesurface of the melt. Preferably, the seed has the same composition asthat of the desired unicrystalline product. A small portion of the seedwill melt and a temperature gradient will be established between thesolid portion of the seed and the melt. The seed is then slowlywithdrawn from the melt while material from the melt solidifies at theinterface between the solid seed material and the melt. The temperaturegradient in the solid phase immediately adjacent to this interface ismaintained at a value which enables desired growth conditions to beobtained (gradient of about lS-QU" C. in a length of about 2 cm). As theseed is withdrawn, an

elongated crystalline mass forms and grows between the seed and themelt. In this melt-grown process, the temperature gradient in the solidphase immediately adjacent to the growth interface is estimated to beabout 100 C. less than the temperature gradient over the solid phaseimmediately adjacent (about 2 centimeters) to the growth interface ofthe prior art fiame-Verneuil process. These temperature gradient valueswere determined indirectly by temperature measurements in the zonesurrounding the seed. This lower temperature gradient greatly reducesthe thermal stresses in the solidified crystal product which lead tointernal strains and crystal misorientations.

Referring now to FIG. I, there is illustrated a crystal growthchamber 1. Ruby or sapphire melt 9 is contained in a crucible 8 which ispreferably fabricated from iridium. A washer 16. preferably iridium,having a central aperture 17 rests on top the crucible 8 and acts as aradiation shield to reduce heat loss from the melt 9. The crucible 8 isbounded on its sides and bottom with insulation 15. The insulation ispreferably thoria (Th;) and serves to: reduce the power required tosustain the melt 9; reduce thermal gradients along the crucible. and todampen temperature fluctuations arising from line voltage fluctuations,convective cooling effects from the atmosphere. as well as otherdisturbances. Hollow tubing 11 forms an aperture through which thetemperature of the bottom of the crucible 8 can be determined by, forexample, a radiation pyrometer focused on the center of the bottom ofthe crucible.

A ceramic washer 4, fabricated from alumina for example, is supported bytubing preferably of thoria (ThO The washer 4 serves as a secondaryradiation shield and to restrict the convective currents of theatmosphere against entering the top of the crucible and reaching thegrowing crystal 7. Thus, it serves to reduce the vertical temperaturegradients in the vicinity of the growing crystal and to augment theeffects of the washer 16.

Sleeve 6, formed of silicon dioxide. for example, serves to contain theinsulation and serves as a part of the insulating assembly surroundingthe crucible 8. The tubing 5 which serves to support the washer 4 alsofunctions as a part of the insulating system.

The crucible 8 and its surrounding insulating assembly rests on aceramic pedestal 12 composed of, for example, zirconium oxide (ZrO Theentire assembly is enclosed in a bell jar 3 sealed to a base plate 13.The base plate 13 is composed of any suitable material such as forexample silicone-bonded fiber glass. The desired gas atmosphere for theinside of the crucible 8 is introduced into sight tube 14 whichcommunicates with tubing 11. The gas exits through the hole 18 in thebell jar 3 through which the seed rod 2 is inserted.

The induction coil 10 which serves to sustain the temperature of themelt 9 is preferably formed of from 7 t0 9 turns of water-cooled coppertubing. The coil is positioned symmetrically with respect to thecrucible to give uniform heating. Electric fields sufiicient to causesparking in argon appear across the work coil in induction heatingdevices such as described here due to the inductive reactance of thecoil at radio frequencies and the high currents passing through thecoil. Thus, to achieve satisfactory operation in monatomic atmospheresit has been found necessary to insulate the coil from the monatomicatmosphere. One means of accomplishing this is to thread the work coil10 and its electrical leads through glass tubing 19 and to circulatedistilled water through such tubing. Alternatively a double walled waterjacket may be interposed between the coil and the atmosphere.

FIG. 2 illustrates the power supply and control system which ispreferably used for the growth of alpha alumina and ruby crystals. Thepower supply is a kw. induction heater, operable at a nominal frequencyof 450 kilocycles with a saturable core reactor to control the poweroutput. The output power can thus be continuously varied from about it)to percent of the rated power. The crucible 8 serves as the load for thepower supply and is loosely coupled to the work coil 10; that is, thecrucibles physical dimensions are small compared to the dimension of thecoil, and as a result the crucible 8 intercepts only a small fraction ofthe magnetic field lines generated by the highfrequency current flowingin the coil 10. Induction heating under these conditions requires alarge number of ampere turns in the work coil.

Control of the output power supply is preferably accomplished by a threemode proportioning controller, fed by a set point unit and deviationamplifier as illustrated. Either of two methods of control is preferred.In the first, a radiation pyrometer is focused on a spot on the bottomof the crucible 8 containing the molten sapphire or ruby, and its outputsignal fed to the control system. The second method utilizes a currentshunt placed in the ground side of the oscillator plate supply of theinduction heating power supply, and the signal generated across thisshunt used for control purposes. Since the power output is roughlyproportional to the square of the plate current, the second methodyields a constant power control as opposed to a constant-temperaturecontrol initially described.

The control signal in either the constant power or temperature controlis fed to a set point unit, a highly stable DC. signal generator with arange from to 20 rnillivolts. The set point unit develops a voltage ofopposite polarity to the control signal. The difference between the twosignals is fed to a deviation amplifier, which amplifies the differenceto provide a strong enough signal to activate the controller. Thecontroller than regulates the output of the power supply to maintain anull signal at its input terminals. The controller has variableproportional band, reset, and rate controls and can thus be adjusted fora variety of response speeds. When using power control, the systempreferably has a response time of about a second. (The response time isthe time required to correct for a deviation from a null inputcondition.) When temperature control is used, a response time of theorder of to seconds is preferable.

The controller output (0-5 ma. DC.) is fed to a magnetic amplifier whichsteps up the output to a current level sufficient to drive the saturablereactors on the power supply, thus completing the control loop.

An auxiliary feedback loop is preferably used to minimize short termfluctuations in output due to line voltage variations. This system isparticularly valuable when temperature control is used in order that afluctuation in power will not eflect a temperature change in thecrucible and melt before corrective action is undertaken. The auxiliaryfeedback loop utilizes a pick-up coil placed near one of the outputleads from the induction heater to give a signal proportional to theoutput current of the power supply. The signal is rectified and fed toan auxiliary winding on the magnetic amplifier illustrated with itspolarity such that it tends to counteract any change in the output fromthe power supply.

With the above described control system, the temperature of the melt canbe maintained constant within a third of a degree at an operatingtemperature of about 2100 C.

The mechanism for pulling the growing crystal from the melt is wellknown in the art. The mechanism allows for the simultaneous pulling androtation of the growing crystal. For operation with monatomic gases asthe atmosphere for crystal growth, it has been found necessary to use along seed rod 2 which extends outside the bell jar 3 as illustrated inFIG. 1 in order to avoid electrical breakdown of the gas occasioned bythe crystal pulling mechanism being at ground potential.

It has been found necessary to follow a very close regimen to grow thesuperior quality crystals of this invention.

In initiating crystal growth, as well as throughout the crystal growingprocess, it is very important to guard against contamination of thesystem. A purge of the atmosphere within the growth regime of allspurious materials is therefore necessary. Thus, high quality crystalswere repeatedly grown when the crucible 8 was charged with high qualityruby or sapphire and the growth apparatus completely assembled a dayprior to growth in order to allow a purge of air and moisture from thegrowth environment by a purging gas such as argon. After the purge, thepower supply is activated and gradually increased to the power settingrequired to melt the charge of saphire or ruby.

After the charge is completely molten, the seed rod 2 is slowly loweredinto the melt and driven downwards for a short time to insure that aclean, freshly melted surface is established for the beginning ofgrowth. The seed rod is usually sapphire because of its availability,but a ruby seed rod would of course suflice. The melt temperature isalso adjusted during this period to reduce the diameter of any solidmaterial which has formed on the seed rod tip to about the diameter ofthe seed rod itself. This is necessitated by the fact that the sapphireseed rod is a very etficient light pipe" and acts as a substantial heatsink, removing suflicient thermal energy to freeze a considerable volumeof alumina from a completely molten charge.

As soon as the system has come to equilibrium at the desiredtemperature, pulling is begun. Pulling rates exceeding /3 inch per hourare generally deleterious to crystal quality. However, the maximumpulling rate can be determined by the onset of the formation of bubbles,voids, or inclusions in the crystals, for it has been found that crystaldeterioration from other factors sets in at growth rates above thelimitation imposed by bubble formation.

In general, it is necessary to monitor the crucible temperature or powersupply output throughout the growing process to obtain crystals of thedesired diameter. At the beginning of the process, unless the initialmelt temperature is precisely right, the temperature of the melt must begradually lowered to bring the crystal diameter to the desired size in areasonable length of time. It is very important that temperature changesespecially in the downward direction, be brought about smoothly andgradually to avoid periods of rapid growth and resultant layers ofbubbles and chromium concentration variation in the crystal.

As the crystal grows in diameter and length, and especially when itbegins to emerge above the washer 16 on the crucible 8, thermal lossesfrom the melt become more pronounced. It is, therefore, necessary toincrease the power to the coil 10 in order to prevent the graduallowering of the temperature of the melt 9.

As has been previously indicated, the growth condition which led to theelimination of bubbles in the growing crystal also produced a crystalmarkedly superior to any known heretofore in other respects. However,many facets are involved in the elimination of bubbles and othercrystalline defects.

The process variable which most significantly affected crystal qualityinsofar as inclusion of bubbles is concerned is the pulling rate. It hasbeen found that crystals without visible defects are repeatedly producedat a maximum pulling rate of 0.25 to 0.50 inch per minute. The crystalswere viewed through a polished flat undcr transverse illumination with15 power microscope.

As another important process variable, it has been found that in thegrowth of alumina based crystals by the present process, thecrystallographic (misorientations, dislocations. etc.) quality isorientation dependent. Using seed crystals having imperfections, it hasbeen virtually impossible to achieve a high quality crystal when growingalong the c-axis (a 0 crystal). This is because any imperfections in theseed will be propagated along the c-axis throughout the length of thecrystal. In order to achieve a high quality crystal of reasonablelength, it has been found that growth should be along an orientationsubstantially different from the c-axis. A 60 orientation, for example,has been found to be quite suitable. Under such an orientation, anyimperfections will be propagated to the sides of the crystal duringinitial growth, leaving the balance of the crystal of high quality.

Another important process parameter is the control of thermal gradients.Of utmost importance is the control of thermal gradients causedprimarily by radiation losses from the melt, the crystal and crucible.One means of reducing the radiation losses is through the use of thewasher 4. Without the washer, the bubble contents of as grown crystalswere at best between 100 and 1000. With the addition of the washer 4,radial thermal gradients, those gradients perpendicular to the verticalaxis of the crucible, were reduced between 30 and 40 C. with asimultaneous reduction of power requirement of about It is felt that alower radial thermal gradient will produce less thermal strain at thesurface of the growing crystal, with a consequent reduction in theoccurrence of defects which serve as nucleation centers for bubbleformation. in addition, it is felt that a lower temperature gradientresults in a lower degree of super saturation of dissolved gases at thegrowth interface thus reducing the opportunity for the formation ofbubbles in the as grown crystal. The washer or lid 4 also improves thethermal stability of the system by: (a) providing a greater degree ofthermal insulation, which will dampen temperature fluctuations; and (b)restricting convection of relatively cool gas from the surroundingatmosphere past the top of the crucible and the growing crystal.

It is of considerable importance to avoid temperature fluctuationsespecially in the downward direction. When the temperature of the meltdrops abruptly as little as one or two degrees Centigrade, a heavy layerof bubbles is produced. The imperfection, can, however, be eliminated byremelting and reforming the crystal in the affected zone.

The composition of the atmosphere within the crystal growing environmenthas a profound effect on crystal quality in several respects includingthe uniformity of the chromium concentration in the crystal, and thedistribution coefiicient or the ratio of the average concentration ofchromium ions in the crystal to that in the melt. In laser crystals,this is of particular significance since the refractive index in ruby isdirectly proportional to the chromium concentration; the lower thevariation in chromium concentration, the lower the variation inrefractive index.

It is well known that ruby material useful in laser applicationsgenerally contains about 0.01 to about 0.2 weight percent chromia. Anexamination of the chromiaalumina phase diagram indicates to one skilledin the art that the segregation coefficient of chromia between theliquid and solid phases is such as to require less chromia in thechromia-alumina melt than would be desired in the solid crystallineproduct. That is, the phase diagram predicts the chromium content ofsolid ruby should at all times be greater than that of the melt withwhich it is in equilibrium regardless of the amount of chromia (C50 inthe melt. In the case of very dilute ruby, that is with the melt chromiaconcentration in alumina approaching zero, the prior art would predictthat the ratio of the chromium concentration in the solid to that in theliquid will be equal to the ratio of the slopes of the liquidus andsolidus curves of the alumina-chromia phase diagram as the chromiaconcentration approaches zero. This value is about 2.1. This is thevalue of the distribution coefficient which would be expected to beexhibited by a growing crystal if growth took place at such a slow ratethat the crystal is in thermodynamic equilibrium with the melt at alltimes during growth.

It is known that a distribution coefficient different from unity impliesa rejection of one component of the system at the growth interface. withthe result that a concentration gradient is built up in the liquid. Theconcentration gradient can only be dissipated by diffusion processes. Itis known that the actual distribution coefficient will asymptoticallyapproach the equilibrium value, 2.1, at low growth rates and highdiffusion rates and will approach unity at high growth rates and lowdiffusion rates. In real crystal growth systems, the observed value willfall between these two extremes. It has been found, however, that evenwhen ruby is pulled at slow rates, for example 0.25 inch/ hour, thediffusion process was sufficiently slow to reduce the observeddistribution coefficient well below the predicted equilibrium value of2.1.

It has also been found that the distribution coefficient varies over awide range and in almost all cases was less than unity. This variationis dependent on the atmosphere within the crystal growing environment.The distribution coefficient observed for growth in pure argon was inthe range of about 0.7 to about 0.8. In an oxidizing atmosphere, thedistribution coefiicient increases as the amount of oxidant in theatmosphere increases. For example, with 20 percent oxygen (0 in anpercent argon atmosphere the observed distribution coefficient was about1.6. When growing ruby in air, this value, 1.6, was not reached in anair-argon atmosphere until the air occupied essentially percent of theatmosphere. The presence of hydrogen in the argon atmosphere also had astartling effect. As the hydrogen approached 20 percent of theargon-hydrogen atmosphere, the observed distribution coefficient wasabout 0.1. The segregation coefficient in nitrogencontaining atmosphereswas about the same as in argon containing atmospheres.

While the reasons for this pronounced atmospheric elfect on distributioncoefficients is not certain, the following is a hypothesis. In stronglyreducing atmospheres, practically all the chromium in the melt isreduced from the trivalent state to some (unknown) lower state, and themelt must be heavily doped to attain a given crystal composition. Inargon atmospheres, the extent of reducing appears to be much lower, butis still sufficient to suppress the distribution coefiicient slightlybelow unity. Only when oxygen is added to the atmosphere is thisreduction prevented and the behavior predictable from the binary phasediagram.

In addition to the above observations, there are a number of seeminglyminor process variables that have drastic effects upon the formation ofbubbles in sapphire and ruby crystals and thus make it difficult toconstruct any simple hypothesis as to their origin. A number of theseinclude the presence or obsence of a ceramic radiation shield (indicatedby reference numeral 4 of FIG. 1) above the crucible, and thecleanliness of the system including the thermal insulation used. Themost important process variable of all, however, is the fact that evenwhen all other conditions have been optimised, there is a limitinggrowth rate above which bubbles are always found.

One variable which has little effect on internal crystal quality is therate at which the seed rod and crystal are rotated. Very high qualitycrystals were grown at rotation rates of about 60 r.p.m. However, onsome occasions it was found that crystals tended to grow off their axisof rotation. This effect can be minimized by slowing the rate ofrotation to about 15 r.p.m.

Using the apparatus and procedure described in the foregoing, highquality ruby crystals, free of any visible inclusions or defects weregrown consistently. Table I below indicates the quality of the rubycrystals grown in argon, and argon-nitrogen atmospheres. As haspreviously been indicated, the bubble content of the ruby crystals correlates with other indicia of quality; as the bubble contentdiminishes the overall quality of the crystal increases. The bubblecontent was ascertained by polishing a flat along one side of the rubycrystal and examining the interior through this polished surface with a15 power microscope under strong transverse illumination. All the 9 rubycrystals grown were of at least one quarter of an inch in diameter. Theorientation of the crystals refers to the angle between the growth orlongitudinal axis of the crystal and the crystallographic c-axis. Theceramic 10 observed in every case. In crystals 11" and c." theconcentration was highest near the seed end and decreased smoothly downthe crystal. The rate of change was about 3.7 percent per centimeter forcrystal [2." In crystal e,"

lid refers to washer 4 of FIG. 1 and unless otherwise a 90 crystal, thegradient was [.8 percent per centimeter indicated the lid was in place.in the opposite direction. Crystal a was cut into sections TABLE IRotation Orien- Growth rate (revo- Crystal tation rate lutions lengthAtmosphere (deg) (iiL/hr.) per min.) (in.) Remarks 60 0.35 60 1%Lilgtlilt mlcrobubblcs present, about 100 to 1,000 in entire crystal; noceramic l 60 0. 5 G0 1% Light microbuhbles present, about 100 to 1,000in entire crystal. 00 0. 25 00 1% Very high quality crystal with novisible defects. 60 0. 25 60 27;; Do. Pure argon 60 0.25 120 lb, Veryhigh quality crystal with less than bubbles throughout the entire erysta0 0. 25 00 2 Very few bubbles present, about 5 to 50 in entire crystal.0 0. 25 0 2/; Very high quality crystal with no visible detects. 60 0.35lb 4 Do. H0 0. 25 00 2 if D0. 5 percent nitrogen in argon. 00 0.35 Lightctlancentration of mlerobubbles present, about. 100 to 1,000 in entirecrysta 15 percent nitrogen in argon... 60 0.35 15 Very high qualitycrystal with no visible defects. 50 percent nitrogen in argon... 00 0.3515 Very light concentration 01' mlcrobubbles present. about 100 to lessthan 1,000

0 Vin egtirle crystal. 1 i h h d w Y 0.5 15 cry ig quality erysta w t novisi le elects. percent numb in 'T 00 035-0. 5 15 The section of thecrystal grown at 0.35 lnchesfhour was a very high quality withessentially no bubbles, the section grown at 0.5 inches/hour had alightt cloncelttration or microbubbles present, about 100 to 1,000 inentire crys a EXAMPLE I A minimum chromium concentration gradient isrequired in the preparation of ruby crystals of high optical quality,since the refractive index of ruby varies with chromium concentration.The uniformity of distribution of the chromium dopant in six rubycrystals grown by the process of this invention was tested. Thesecrystals were fabricated into windows with flat faces both parallel(longitudinal) and perpendicular (transverse) to the crystal axes toallow examination for both axial and radial chromium concentrations. Theuniformity of distribution of the chromium dopant in the ruby crystalswas determined by transversing a sharply collimated light beam at 5470A. across the window samples. 5470 A. is the peak of one of theabsorption bands of ruby. The dimensions of the light beam where itpassed through windows was about 0.1 mm. in order that chromiumfluctuations over small and samples from its top, middle and bottomregions submitted for chromium analysis by chemical methods. Thechromium concentration in this crystal varied over a 2 inch length from0.042 percent at the top to 0.037 percent at the bottom agreeing quitewell with the data on crystals b" and 0.

EXAMPLE II distances could be ascertained. Table it below lists thefflcfi- The same etchmg techmqye was rc d to y more important growthconditions with the crystal quality CPI/31315 grown y [he flame fuslonor Ve'mwll P Q for th crystals, Th li t d parameters h h Same The flamefusion crystals yielded dislocation densltles of meaning as in Table I.severalX 10 per square centimeter. The average disloca- TABLE IIOrientation Growth Rotation Crystal Atmosphere ttlcg.) rate rate Remarksa Pure urgon H. 00 0, .15 6 Poor grade crystal, large bubbles present(bubbles due to equipment failure) Crystal fabricated into a transversewindow. 60 0. 25 120 Very high uality crystal, about 5 to less thanbubbles ill entire crystal.

Crystal la ricated into longitudinal and transverse window. on 0.25 Do.4

0 0. 25 60 High quality crystal, about 5 to 50 bubbles in entirecrystal. Crystal fabricated into longitudinal and transverse windows. 000. 25 00 Very high quality crystal, no visible detects. Crystalfabricated into longitudinal and transverse windows. I 50 percentnitrogen ill argon. 00 0.85 15 Crystal bad light concentrations ofmicrobubbles present, about 100 to 1,000

The radial variation in chromium concentration for crystals :1," c and eis quite small with a maximum variation over 12 mm. of about 0.001percent chromia (Cr O These three crystals have less radial chromiumvariation than any known crystals. Relatively larger variations wereobserved in crystals b," (1" and T which differed in growth parametersfrom the other crystals by having a fast rotation rate, 120 r. .m.; azero degree orientation; and a nitrogen in argon atmosphererespectively.

The same spectrophotometric technique was used for the determination oflongitudinal chromium concentration variations in crystals b, c, d ande. A gradual variation in chromium concentration along the crystal wasill entire crystal. Crystal fabricated into transverse window.

tion counts for the crystals pulled in accordance with this inventionwere of the order 10 to 10 per square centimeter with some areas givingetch pit counts of the order of 100/cm.

EXAMPLE III 1 l dislocations, propagate parallel to the c-axis of rubyand sapphire crystals pulled from the melt. Observations of dislocationdensities in 60 crystals indicate that imperfections are of a relativelyhigh magnitude near the seed 12 joules were observed. This output pulsecoupled with the smaller beam diveregnce of the instant crystals allowedholes to be burned in exposed X-ray film from a distance of up to 5feet. This was achieved even though the end of the crystal and diminishdramatically as the dis- 5 laser output beam was not focused. Thethreshold entance from this region increases. Thus, dislocations andergies for lasing were iniformly quite low for the pulledmisorientations in the seed would grow oil to the side ruby rods,remaining more consistently so than the flame of the crystal and soonterminate on its surface leaving fusion ruby rods. The threshold pumpingenergies for the the remaining portions of the crystal free from theinflurods which were annealed were in all cases lower than ence of theseed crystal. those for the corresponding unanncaled rods, the averageEXAMPLE 1V difference being about 20 percent.

In addition, the far field patterns of the output beams The mostinformative technique used for evaluation from the pulled ruby rods werequite symmetrical and of the optical homogeneity of laser rods of rubygrown circular in shape even though the beam was not focused. inaccordance with this invention is Twyman-Green interterometry. Thistechnique measures directly the variations EXAMPLE VI in the opticalpath (physical length multiplied by refrac- About 75 grams of cleanedchunks of alumina crystals tive index) through the rod. When the endfaces of the were placed in a tungsten crucible having an insidediamlaser rod are ground truly parallel, the Twyman-Green eter of 1%inches, a wall thickness of -inch and a interferometer provides anaccurate determination of the 20 height of 1 /2 inches. The crucible wasplaced within a variation of the refractive index over the cross-sectionof 14 turn induction heating coil having an ID. of 1% the rod.Alternating light and dark bands, interference inches. The cruciblestood on a pedestal containing packed fringes, will appear where changesin refractive index thoria powder while the space between the coil andthe occur. Ad acent fringes represent variations in the optical cruciblewas also packed with thoria powder. This entire path of one-half thewavelength of light used. The total apparatus was enclosed in a glassbell jar having an variation across a given rod 15 then given by:aperture at its top. An inert argon atomsphere containing N) about 50volume percent hydrogen was maintained inside the bell jar. Theinduction heating coil was energized from a well known R-F inductionheating unit and the Where N Is the total number of fringes observed, Athe power was increased until the induced current in the wavhlehgth ofthe hght h h length t rod tungsten crucible heated it to a white heat.Conductive Cehhmethrs! the vahahoh h refrachvh heat from the tungstencrucible then melted the alumina All of the 60 rods showed two fringesor less variation. chunks to f a melt A unicrystamne alpha alumina Smcethesh f h m to 6 long the maxl seed having a size of about 0.10-in. dia.was lowered mum vghahoh 1h hf Index of the Order of through the aperturein the bell jar until it contacted the 2X10- or less. In addition, thefringes present Wt?"j surface of the melt. The seed was then withdrawnfrom in a regular pattern. Interferograms of flame fusion crysthe meltat about Pinch per hour f 1 hours h tals show a materially greaternumber of interference power to the induction coil and thc n rate werefuhgesg Often no regular Patterh' justed from time to time in order toobtain a substan- Schhereh hhotographs of the why Crystals analyzed 40tially constant cross-section boule product. A final elonh theTwyman-(ieen h f were also taken gated boule having a diameter of about/z-inch and m order to determine variat ons in theinrefractive index.length of about 2 incl-3S was thus Obtained (volume T vanatlon light h aSchhereh phmqgraph of 0.39 cubic inch). This boule was massivenon-granular is directly related to the derivative of the refractiveindex. r unicryswuine alpha alumina (corundum) having The Schlierenphotographs of the 60: pulled ruby rods 40 proved OpticalCharacteristics showed very low variation in refractive index and thusindicated an extremely high quality crystal. Flame fusion- EXAMPLE VIIrown rub showed considerable variations in re ertics a anay p p A chargeof 99.5 grams of almtna crystals and 0.5

yzed by this technique.

gram of chromia (0.5 welght percent Cr O was placed EXAMPLE v in atungsten crucible and melted in a manner similar Lasing tests wereperformed on ten laser rods grown to that hescrihed hl Example A h ShedF in accordance with this invention. Table III below shows 18] having 3of {ibout was lhsermd h the most important growth parameters of theselaser the melt and than whhdfawh at a rate of about lA'lhch rods. Theprocess parameters have the same meaning P hOUFZ After pp l y 4% hours,a boule indicated previously for the other tables. As indicated inch indiameter and 1% in. long was grown volume in the table, four of thecrystals were annealed. of 0.086 cubic in.). This massive, non-granularunicrystal- TABLE III Rotation Orienta- Growth rate (revotion ratelutions (deg) (UL/ht.) pcrniiu.) Atmosphrrc Remarks 0.25 60 Pure argonVery good crystal, less than 10 bubbles throughout. 60 0.25 do Do. 60 O.25 Very good crystal, no visible defects. 60 035 D0. 60 t). Versb goodcrystal, no visible defects; crystal annealed. 60 0. o.

t] 0. 25 Very good crystal, about 5 to 50 bubbles in entire crystal;crystal annealed. t] 0. 25 Very good crystal, no visible defects. 900.25 (it) do. Do. 00 t). 25 00 .do Very good crystal, no visibledefects; crystal annealed.

The results of the lasing tests indicate that the above tested crystalsperformed considerably better than prior art ruby crystals. The beamdivergence of a majority of the crystals was about half that of a highquality flame line ruby boule had improved optical characteristics.

EXAMPLE VIII A charge of alpha alumina (corundum) crystals wasfusion-grown laser rod. Output pulses greater than 3.5 placed in aniridium crucible and heated with an oxygen- 13 hydrogen flame. No inertatmosphere was used. An alpha alumina seed crystal was withdrawn fromthe melt to produce a massive, nongranular unicrystalline alpha aluminaboule product having improved optical characteristics.

Massive unicrystalline alpha alumina and chromia alpha alumina materialsobtained by techniques similar to those described above were evaluatedfor optical properties. Examination of the crystals in cross-Polaroidfilters indicated no internal strain or misorientation (no subgrainboundaries). Schultz-Wei X-ray photographs show no crystalmisorientation. The limit of detection of this technique isapproximately 1 min. of arc misorientation. Furthermore, the dislocationdensity of this material is less than about l /sq. cm. whereas prior artflame-fusion grown alumina had dislocation densities of greater thanabout IO/sq. cm. The dislocation density is an indication of internalstrain of the crystal and thus the lower the dislocation density, thebetter the material.

The above discussion is directed primarily at the production of massiveunicrystalline alpha alumina products useful in laser equipment. Itshould be understood that the products of the present invention can alsobe used in other optical applications as well as for wear-resistantapparatus, such as textile fiber guides, and for gemstones. Such utilityis well known in the art for unicrystalline alpha alumina material.

What is claimed is:

1. A process for the production of massive unicrystalline alpha aluminawhich is substantially free of internal strain and crystalmisorientation in the as-grown state which comprises:

forming a melt of alumina by heating same to a temperature of at least2040 C.;

inserting a seed rod of alpha alumina into the melt;

maintaining an atmosphere over the melt which is substantiallychemically inert to the melt;

withdrawing the seed rod from the melt such that alumina material issolidified and crystallized on the seed rod to form a massiveunicrystalline product of increasing length, the withdrawal rate of theseed rod from the melt being sufficiently limited such that bubbles donot form in the massive unicrystalline product;

and controlling the temperature of the seed rod and massiveunicrystalline product such that at least a portion of the radiantenergy emitted therefrom is supplied to the atmosphere over the melt. 2.A process for the production of massive unicrystalline ruby which issubstantially free of internal strain and crystal misorientation in theas-grown state which comprises:

forming a melt of alpha alumina and chromia by heating same to atemperature of at least 2040 C.;

inserting a seed rod into the melt, the seed rod selected from a classof materials consisting of alpha alumina and ruby;

maintaining an atmosphere over the melt which is sub stantiallychemically inert to the melt;

withdrawing the seed rod from the melt such that ruby material issolidified and crystallized on the seed rod to form a massiveunicrystalline product of increasing length, the withdrawal rate of theseed rod from the melt being sufiiciently limited such that bubbles donot form in the massive unicrystalline product;

and controlling the temperature of the seed rod and massiveunicrystalline product such that at least a portion of the radiantenergy emitted therefrom is supplied to the atmosphere over the melt.

3. The process claimed in claim 2 wherein the atmosphere is argon andthe seed rod is withdrawn at a rate not exceeding about 0.50 inch perhour.

4. The process claimed in claim 3 wherein the temperature of the melt isnot varied over two degrees centigrade.

5. The process claimed in claim 2 wherein the distribution coefficientbetween the melt and the ruby material is increased by adding anoxidizing gas to the atmosphere above the melt.

6. The process claimed in claim 2 wherein the distribution coetiicientbetween the melt and the ruby is lowered by adding a reducing gas to theatmosphere above the melt.

7. A process for the production of massive unicrystalline alpha aluminawhich is substantially free of internal strain and crystalmisorientation in the as-grown state which comprises:

forming a melt of alumina and chromia by heating same to a temperatureof at least 2040 C.; inserting a seed rod of alpha alumina into themelt; flowing an oxidizing gas over the surface of the melt and thenremoving said gas from the vicinity of the melt surface;

withdrawing the seed rod from the melt such that alumina material issolidified and crystallized on the seed rod to form a massiveunicrystalline product of increasing length;

and controlling the temperature of the seed rod and massiveunicrystalline product such that at least a portion of the radiantenergy emitted therefrom is supplied to the atmosphere over the melt.

References Cited UNITED STATES PATENTS 3,224,844 12/1965 Gerthsen 2330l3,519,394 7/1970 Petit-Le Du et a1 23-301 3,527,574 9/1970 La Belle, Jr.23-301 3,591,348 7/1971 La Belle, Jr. 23301 3,595,803 7/1971 Dugger23305 FOREIGN PATENTS 935,390 8/1963 United Kingdom 23-305 15,901 7/1968Japan 23305 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, AssistantExaminer US. Cl. X.R.

